Metal pipes practically used as laid underground have been provided with tar or bitumen coating since long ago for the purpose of corrosion prevention. When the coating of a metal pipe is scratched, however, the corrosion of the metal pipe progresses from the scratched portions. For the purpose of solving such a corrosion problem, the following corrosion prevention is widely applied: a metal coating having a higher ionization tendency than the ionization tendency of the material of a metal pipe is formed on the surface of the metal pipe, the metal coating undergoes the generation of a sacrificial anode function due to the ionization tendency difference, and thus the metal coating prevents the corrosion starting from the scratched portions. Zinc is a typical metal having such a sacrificial anode function. A zinc coating is formed on the surface of a metal pipe such as an iron pipe by plating or spraying. Such a coating is used, as it is, as the outermost surface layer, or as it is further overcoated with another layer. Zinc has a high ionization tendency; the electrochemical potential difference between iron and zinc is large, and accordingly, in a case where zinc is used in combination with an iron-base metal, even when some scratches are caused in the coating, the sacrificial anode function is displayed and the corrosion in the scratched portions can be suppressed. In the case of cast-iron pipes widely used as water and sewage pipelines, the coating is covered with a polyethylene sheet referred to as a polyethylene sleeve to block the coating from the external environment, and consequently the corrosion prevention effect is further enhanced.
However, zinc has a high ionization tendency, and hence zinc hardly maintains the sacrificial anode function over a long period of time. As a solution to solve this problem, the increase of the zinc coating amount is an effective technique. However, in this case, in addition to the increase of the material cost, the work time is increased and the production efficiency is also degraded.
Alternatively, as another method, a zinc-aluminum alloy is used (WO94/19640) as the case may be. The addition of aluminum alleviates the ionization, and consequently the retention time of the sacrificial anode effect is maintained over a longer period of time.
However, as for aluminum, health concern is raised from some viewpoints, and the safety of aluminum as a material applied to drink water supply pipes is not established. For example, in a pipe joint having an inserting-receiving structure in which the inside of a socket formed at one end of a pipe receives a spigot formed at one end of another pipe, the outer surface of the spigot is brought into contact with tap water, thus causing a possibility that aluminum is eluted from the outer surface of the spigot.